ANN ARBOR—Slightly more than 4 percent of people given death sentences in the United States are innocent, according to new peer-reviewed research led by a University of Michigan expert.

The finding shows that the number of innocent people sentenced to death is more than twice the number of inmates exonerated and freed by legal action, according to a study published today in the Proceedings of the National Academy of Sciences.

No process of removing potentially innocent defendants from the execution queue can be foolproof. "With an error rate at trial over 4 percent, it is all but certain that several of the 1,320 defendants executed since 1977 were innocent," the study concludes. The researchers said this percentage is "a conservative estimate."

About 1.6 percent of those sentenced to death since 1973 in the U.S.—138 prisoners—have been exonerated and released because of innocence. But many other innocent capital defendants are missed.

"The great majority of innocent people who are sentenced to death are never identified and freed. The purpose of our study is to account for the innocent defendants who are not exonerated," said Samuel Gross, a professor in the U-M Law School and the study's lead author.

A major reason that many innocent capital defendants are not exonerated is that they are removed from death row but remain in prison. Lawyers, courts and governors focus a great deal of attention and time on reviewing the cases of prisoners who are on death row in order to reduce the risk of executing innocent people. But most death row prisoners have their sentences reduced to life in prison after appeals.

When that happens, the threat of execution is removed and far less time and resources are devoted to seeking out cases of possible innocence. As a result, an innocent defendant who might well be exonerated and released if he remained on death row for many years is likely to die in prison if he is resentenced to life.

The study's other authors are Barbara O'Brien, professor at Michigan State University College of Law; Chen Hu, a senior biostatistician, American College of Radiology Clinical Research Center and Edward H. Kennedy of the University of Pennsylvania School of Medicine.